1. Field of the Invention
The present invention relates to an automatic focus detection system for use in camera, and more particularly to an automatic focus detection system, in which the focusing condition of an objective lens with respect to an object is detected with illuminating the object, if the brightness of the object is lower than a predetermined level.
2. Description of the Prior Art
FIG. 1 shows a known construction of conventional automatic focusing camera in which the amount of defocus of the objective lens is calculated and the objective lens is automatically focused on the basis of the calculated result. In the figure, the light transmitted through an objective lens 601 is passed through a half-mirror 602, and then reflected by a reflection mirror 603 towards an image sensor 604 including a plurality of light receiving elements. The image sensor 604 receives an image of an object to be photographed by the objective lens 601, and the amount of defocus of the objective lens 601 is calculated in accordance with the position of the image formed on the image sensor 604, or with the contrast of the image. The objective lens 601 is automatically focused on the basis of the calculated amount of defocus. If the brightness of the object to be focused is lower than a predetermined level, a light source 605 provided in a camera body is actuated for projecting light towards the object through the projection lens 606. Here, a pattern plate, having a plurality of transparent portions and a plurality of opaque portions, is provided in front of the light source 605 for projecting a contrast pattern on the object, in order to make higher contrast on the object.
In such construction, the focus detection area on the object, in which the focusing condition of the objective lens is detected, is determined to be an area positioned on an optical axis of the objective lens. Such focus detection area is normally determined to be located so that it has a predetermined extent in a direction perpendicular to the optical axis of the objective lens with having its center arranged on the optical axis. Therefore, the position of the center of the focus detection area is kept on the optical axis even if the focal length of the objective lens is changed, although the size of the focus detection area on the object is changed in accordance with the change of the focal length of the objective lens. Thus, the light projected by the light source and the projection lens illuminates a predetermined area including the optical axis in spite of the change of the focal length of the objective lens.
However, a multi-point focus detection device having a plurality of focus detection areas on the object has at least one focus detection area whose center is not positioned on the optical axis. Such focus detection area is called "off-axis focus detection area" hereinafter. In such multi-point focus detection device, the position of the center of off-axis focus detection area is changed in addition to the change of its size, in accordance with the change of the focal length of the objective lens. The conventional auxiliary light projection system having the light source and the projection lens as shown in FIG. 1, however, projects the light in a predetermined and fixed direction within a predetermined and fixed angle. Therefore, if the conventional auxiliary light projection system is used with the multi-point focus detection device, the off-axis focus detection area may not be illuminated sufficiently.
To solve such problem, it is necessary to provide a plurality of light sources or a light source having a wide light emitting region for illuminating wider area. It causes, however, more consumption of electric energy supplied by a power source of the camera, resulting the deterioration of compactness thereof. The automatic focusing camera having a plurality of light sources for illuminating the object has been proposed, for example, in Japanese Laid-Open patent application Nos. 201215/1983 and 168111/1985. However, they fail to disclose the solution of the problem caused by the change of the position of the focus detection area due to the change of the focal length of the objective lens.